1. Field of the Invention
The present invention relates to an optical fiber having an end face which allows optical coupling with another optical fiber through its physical contact as well as to a method for coupling such optical fibers and, more particularly, to an optical fiber at an output end of a high power optical amplifier, to an optical fiber arrangement for optical transmission, to an optical amplifier including such a fiber, and also to an optical transmission system including such a fiber.
2. Description of the Related Art
FIG. 8 shows an outline of an optical fiber connector. More in detail, an optical connector 800 functions to bring two optical fibers 810 into physical contact for their optical coupling. To this end, a user mounts plugs 81 onto respective end faces of the two optical fibers 810 to be connected, and then tightly screws the plugs into associated screws 802 of an adapter 80 provided at its both sides, whereby the two optical fibers are accurately positioned and coupled. In this case, since the adapter 80 is formed therein with engagement hollows 801, the engagement hollows 801 are engaged to receive associated engagement nails 812 of the plugs 81 to thereby prevent the adapter 80 from being rotated.
The user strips off skin layers of the optical fibers 810 to expose cores (not shown) thereof, and then covers the cores with the plugs 81, at which time the user inserts the fiber cores into associated ferrules 4 of the plugs 81.
FIG. 9 is a cross-sectional view of a major part of a prior art optical connector for optical fibers. The optical connector has such a structure as to be explained below. That is, the refractive index distribution of optical fiber is determined by dopant distribution. The optical fiber is provided in its center part with a core 2, which has a high refractive index and around which a cladding 3 having a low refractive index. Provided further around the cladding is the ferrule 4 (which outer peripheral part is omitted in the drawing) for reinforcement. Thereafter, for the purpose of preventing light reflection at the end face 5 of the fiber, the end face 5 is subjected to an accurate polishing operation to provide a reflection of -40 dB or less at a connection. The silica single-mode optical fiber conventionally used, when transmitting light having a wavelength of 1.55 .mu.m therethrough, produces a mode field 10 which has a diameter of about 10 .mu.m that is the same even in the interior portion of the fiber and even at the end portion thereof.
JP-A-7-128544 discloses an invention in which the mode field of an optical fiber at a connection part between a light waveguide and the optical fiber is locally expanded. This invention, which is directed to permanent splice based on melt or adhesion between the waveguide and optical fiber, is different from the present invention which is directed to the repetitive removable connection based on the connector.
The emergence of optical amplifier expects that an optical fiber having a high level light output of 20 dBm or more will be used in an optical transmission system. However, it has been recently found that, when dust particles (which will be called merely dust, hereinafter) floating in the air are deposited on the light-exit end face of a usual single-mode optical fiber having a light output of 20 dBm or more, the light power converges into the dust, thus resulting in that the end face having the dust deposited thereon bakes and starts to melt.
The process leading to the end face melt is as follows.
The connecting operation of the optical connector involves invasion of dust onto interfaces between the end faces of the optical fibers to be connected. When a light signal is transmitted through the optical fibers, the light is irradiated on the dust invaded into the connected parts of the optical connector. Absorption of the light power into the dust produces heat. The end face melt of the optical fibers is considered to be a phenomenon which is caused by the fact that the produced heat increases the temperature of the end portions of the optical fibers to the melting point or more of the material of the fibers.
When the prior art single-mode optical fiber shown in FIG. 9 has a light power of 20 dBm (100 mW), its power density becomes 1300 MW/m.sup.2. Assume that dust deposits on the end face of the silica fiber and the light passing through the fiber is all absorbed into the dust to be changed to heat. Then since silica glass has a thermal conductivity of 19.0 W/m/K (at 100 C.), a specific heat of 1.04 J/g/K and a density of 2.22 g/cm.sup.3 ; a time taken for the surface of the silica glass to reach its melting point of 1600 C. is 30 sec. That is, the dust remaining at the connector causes the end face to melt in a moment.
In particular, when an abrupt increase of light input power is applied to an optical amplifier, this causes generation of an abnormal high peak light power as a surge, which disadvantageously leads to the fact that the light exit end face of the optical fiber tends to easily bake and melt.
Further, in the case of an optical transmitter having an optical fiber and an optical amplifier built therein, the bake and melt of the end face of the optical fiber results in a failure of the optical transmitter. In the case of an optical transmission system having such an optical transmitter built therein, the bake and melt of the end face of the optical fiber unfavorably leads to a reduction in the reliability of the optical transmission system.
A method for preventing the bake and melt of the end face 5 of the optical fiber is considered to previously connect the adapter 80 and the plugs 81 in a clean room to avoid deposition of dust on the end faces of the optical fibers. However, it is highly difficult to previously connect a transmission line and such a discrete device as an optical amplifier. In addition, the optical fiber, optical amplifier and optical transmission system are often installed at a usual office room, so that temporary removal of the optical connector at the time of installation or maintenance results in deterioration of the reliability of the overall transmission system.
As explained above, in this way, the prior art optical fiber, optical amplifier and optical transmission system, in order that the optical fiber to transmit high power output light, requires the light exit and entrance ends of the optical fiber to be kept clean. To this end, interconnection between the adapter and plugs requires a clean room. In addition, once disconnection and re-connection of the connector at the time of the installation and maintenance disadvantageously involves a possibility that the bake and melt may take place at the end face of the optical fiber. The optical amplifier, in particular, tends to easily bake and melt at the end face of the optical fiber, thus deteriorating the reliability of the optical transmission system.